Fine carbon fiber and method for producing the same

ABSTRACT

A fine carbon fiber having a multilayer structure having stacked cylindrical carbon sheets and a center axis having a hollow structure. The fine carbon fiber has an outer diameter of 2 to 300 nm and an aspect ratio of 10 to 15,000, and at least one cylindrical carbon sheet layer among the multiple layers is folded at an end part of the carbon fiber and continued to another cylindrical carbon sheet. The folded and continued cylindrical carbon sheets form a cylindrical structure opened at the end part.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofProvisional Application 60/267,177 filed Feb. 8, 2001 pursuant to 35U.S.C. §111(b).

FIELD OF THE INVENTION

[0002] The present invention relates to a fine carbon fiber having aspecific structure and also relates to a production process and anapplication of the carbon fiber. More specifically, the presentinvention relates to a fine carbon fiber suitable as a filler forcomposite materials of resin, rubber or the like, a semiconductormaterial, a catalyst and a field emission material, and also relates toa production process thereof.

BACKGROUND OF THE INVENTION

[0003] Carbon fiber is used in various composite materials because ofits excellent properties such as high strength, high elastic modulus andhigh electric conductivity. With the progress of electronic technologiesin recent years, carbon fiber is expected to be used as an electricallyconductive resin filler for electromagnetic wave-shielding materials orantistatic materials or as a filler in a resin for use in anelectrostatic coating by using not only the excellent mechanicalproperties of the carbon fiber, which have been heretofore utilized, butalso the electrical conductivity of the carbon fiber or carbon material.Furthermore, the carbon material is expected to be used as a fieldemission material for a flat display and the like using of itsproperties such as chemical stability, thermal stability and finestructure.

[0004] Conventional carbon fiber is produced as a so-called organiccarbon fiber which is obtained by heat-treating and carbonizing fibersuch as PAN(polyacrylonitrile), pitch- or cellulose-based fiber. In thecase of using this carbon fiber as a filler for fiber reinforcedcomposite materials, the carbon fiber is preferably reduced in itsdiameter or increased in its length, thereby enlarging the contact areawith the matrix so as to elevate the reinforcement effect. Furthermore,for improving the adhesion to the matrix, the surface of the carbonfiber is preferably not smooth and is roughened to some extent bysubjecting the surface of the carbon fiber to a surface treatment suchas oxidation by exposure to air at a high temperature or coating or thelike.

[0005] However, the organic fiber used as the starting material of thiscarbon fiber has a diameter of approximately from 5 to 10 μm andtherefore, the produced carbon fiber cannot have a small diameter and islimited in the ratio of length to diameter (i.e., aspect ratio). Underthese circumstances, there is a demand for the development of carbonfiber having a small diameter and a large aspect ratio.

[0006] When resin is used for an automobile body or when resin, rubberor the like is used for an electronic device, the resin, rubber or thelike is required to have electrical conductivity comparable to metal.Accordingly, there is a demand that the carbon fiber used as a fillermaterial also has higher electrical conductivity so that therequirements demanded in various electrically conductive coatingmaterials, electrically conductive resin and the like, can be satisfied.

[0007] In order to have higher electrical conductivity, the carbon fibermust be graphitized and thereby improved in electrical conductivity. Toimprove electrical conductivity, the carbon fiber is usually graphitizedat a high temperature. However, even by graphitization, the carbon fibercannot have electrical conductivity comparable to metal. If the amountof carbon fiber blended is increased to compensate for this insufficientelectrical conductivity, the obtained composite materialdisadvantageously decreases in workability and mechanical properties.Therefore, it is necessary to further improve the electricalconductivity of the carbon fiber itself or enhance the strength byreducing the diameter.

[0008] With respect to the use as a field emission material, studieshave heretofore been made on the field emission by the Spint method.However, the production process by this method involves many steps andalthough the carbon fiber used for the electron emitting part isconventionally processed to have a needle-like tip using Mo or the like,the chemical stability and the thermal stability are not sufficientlyhigh as an electron emitting material of a display.

[0009] In the late 1980's, studies have been made on vapor grown carbonfiber (hereinafter simply referred to as VGCF) of which the productionprocess is utterly different from that of the organic fibers.

[0010] This VGCF is known to be obtained from the vapor-phase thermaldecomposition of a gas such as hydrocarbon in the presence of an organictransition metallic catalyst, and a carbon fiber having a diameter offrom hundreds of nm to 1 μm is obtained.

[0011] For example, a method where an organic compound such as benzeneis used as a starting material and an organic transition metal compoundas a catalyst, such as ferrocene, is introduced into a high-temperaturereaction furnace together with a carrier gas to produce VGCF on asubstrate (JP-A-60-27700 (the term “JP-A” as used herein means an“unexamined published Japanese patent application”)), a method ofproducing VGCF in the free state (JP-A-60-54998), and a method ofgrowing VGCF on a reaction furnace wall (Japanese Patent No. 2778434)are known.

[0012] According to these production processes, carbon fiber suitable asa filler material by having a relatively small diameter, an excellentelectrical conductivity and a large aspect ratio can be obtained and inpractice, carbon fiber having a diameter of approximately from 100 to200 nm and an aspect ratio of approximately from 10 to 500 ismass-produced and used as an electrically conductive filler material infillers for resin or in additive materials for lead storage batteries.

[0013] The VGCF is characterized by its shape and crystal structure.This fiber has a structure such that carbon hexagonal network surfacecrystals are stacked like annular rings to form a cylindrical shape, andthe center part thereof forms a very narrow hollow moiety.

[0014] However, on a mass-production scale, VGCF having a small diameterof less than 100 nm cannot be produced.

[0015] Iijima, S., 1991, Nature, 354, 56, have discovered a multi-layercarbon nano-tube obtained from soot after the evaporation of a carbonelectrode by arc discharge in a helium gas and this carbon fiber has adiameter smaller than that of VGCF. This multi-layer carbon nano-tube isa fine carbon fiber having a diameter of 1 to 30 nm, where, similarly toVGCF, carbon hexagonal network crystals are stacked like annular ringscentered in the fiber axis and closed to form a cylindrical shape andthe center part thereof has a hollow moiety.

[0016] This method using arc discharge is, however, not suitable formass-production and not implemented in practice.

[0017] The vapor-phase process has a possibility of producing a carbonfiber having a large aspect ratio and a high electrical conductivity andstudies are being made to improve this process with an attempt toproduce a carbon fiber having a smaller diameter. U.S. Pat. No.4,663,230 and JP-B-3-64606 (the term “JP-B” as used herein means an“examined Japanese patent publication”) disclose a cylindrical carbonfibril comprising graphite and having a diameter of about 3.5 to about70 nm and an aspect ratio of 100 or more. The structure thereof is suchthat continuous layers of regularly oriented carbon atoms are disposedconcentrically about the axis of the cylinder to form multiple layers,the C-axis of each carbon atom layer is substantially orthogonal to thecylinder axis, a thermal carbon coating deposited by thermaldecomposition is not contained in the entirety, and the surface issmooth.

[0018] JP-A-61-70014 discloses a vapor grown carbon fiber having adiameter of 10 to 500 nm and an aspect ratio of 2 to 30,000, where thethickness of the pyrolytic carbon layer is 20% or less of the fiberdiameter.

[0019] These carbon fibers all have a smooth surface, and therefore, arepoor in adhesive property, wettability and affinity, and when used as acomposite material, the surface of the carbon fiber must be treated, forexample, by thorough oxidation. Furthermore, when used as a fieldemission material, the tip of the carbon fiber must be thinned.

SUMMARY OF THE INVENTION

[0020] An object of the present invention is to provide a fine carbonfiber capable of serving as a filler material having high electricalconductivity and a diameter of less than 400 nm, preferably from 2 to300 nm, and exhibiting good adhesive property to resin or the like.

[0021] A further object of the present invention is to provide such finecarbon fibers on a mass-production scale.

[0022] Another object of the present invention is to provide achemically and thermally stable field emission material having anexcellent electron emission property and a long life.

[0023] The present inventors have discovered a new fine carbon fiberhaving a structure different from conventional carbon fibers, includingthe production process thereof. The present invention provides thefollowing embodiments.

[0024] (1) a fine carbon fiber comprising cylindrical carbon sheetsstacked to form a multilayer structure with the center axis thereofhaving a hollow structure, the fine carbon fiber having an outerdiameter of 2 to 300 nm and an aspect ratio of 10 to 15,000, wherein atleast one cylindrical carbon sheet layer among the multiple layers isfolded at an end part of the carbon fiber and continued to anothercylindrical carbon sheet and the folded and continued cylindrical carbonsheets form a cylindrical structure opened at the end part;

[0025] (2) the fine carbon fiber as described in (1), wherein the foldedand continued cylindrical carbon sheets are present in the peripheralpart of the multilayer structure;

[0026] (3) the fine carbon fiber as described in (2), wherein acylindrical carbon sheet closed at the end part is present inside thecylindrical structure formed by the folded and continued cylindricalcarbon sheets;

[0027] (4) the fine carbon fiber as described in (3), whereincylindrical carbon sheets folded and continued at the end part to form acylinder opened at the end part of the carbon fiber are present furtherinside the cylindrical carbon sheet closed at the end part;

[0028] (5) a fine carbon fiber having an outer diameter of 2 to 300 nmand an aspect ratio of 10 to 15,000, wherein the fine carbon fiberdescribed in any one of the (1) to (4) occupies about 5% by mass or moreof the fine carbon fibers;

[0029] (6) a fine carbon fiber having an outer diameter of 2 to 300 nmand an aspect ratio of 10 to 15,000, wherein the fine carbon fiber asdescribed in any of (1) to (4) occupies from about 5 to about 90% bymass of the fine carbon fibers;

[0030] (7) a fine carbon fiber having an outer diameter of from 2 to 300nm and an aspect ratio of from 10 to 15,000, wherein when observedthrough a transmission electron microscope, the fine carbon fiberdescribed in any one of the (1) to (6) occupies from about 3 to about80% by volume in the fine carbon fibers;

[0031] (8) the fine carbon fiber as described in any one of the (1) to(7), wherein the fine carbon fiber is vapor grown carbon fiber;

[0032] (9) the fine carbon fiber as described in any one of the (1) to(8), wherein the carbon fiber comprises a boron atom;

[0033] (10) the fine carbon fiber as described in any one of the (1) to(9), wherein carbon atoms of the carbon fiber are partially displaced byboron atoms;

[0034] (11) a process for producing the fine carbon fiber described inany one of the (1) to (10), comprising heat-treating fine carbon fiberhaving an outer diameter of 2 to 300 nm and an aspect ratio of 10 to15,000 and having a multilayer structure formed by cylindrical carbonsheets stacked one on another, with the center axis having a hollowstructure;

[0035] (12) the process for producing the fine carbon fiber as describedin (11), wherein the heat-treatment temperature is from about 2,000 toabout 3,500° C.; and

[0036] (13) the process for producing the fine carbon fiber as describedin (11) or (12), wherein a boron compound is mixed with fine carbonfiber having an outer diameter of from 2 to 300 nm and an aspect ratioof from 10 to 15,000 and having a multilayer structure formed bycylindrical carbon sheets stacked one on another, with the center axishaving a hollow structure, and the mixture is heat-treated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a schematic cross section for describing the structureof a conventional fine carbon fiber.

[0038]FIG. 2 is a schematic cross section for describing the structureof the fine carbon fiber of present invention.

[0039]FIG. 3 is a schematic cross section for describing the structureof the fine carbon fiber of present invention.

[0040]FIG. 4 is a schematic cross section for describing the structureof the fine carbon fiber of present invention.

[0041]FIG. 5 is a schematic view showing an apparatus for use in theproduction described in the Example of the present invention.

[0042]FIG. 6 is a transmission electron microscope photograph(magnification: 2,000,000 times) of a conventional fine carbon fiber.

[0043]FIG. 7 is a transmission electron microscope photograph(magnification: 2,000,000 times) of a heat-treated fine carbon fiber ofthe present invention, which is heat-treated using a boron compound.

DETAILED DESCRIPTION OF INVENTION

[0044] The present invention is described in detail below.

[0045] In the process of making investigations for a fine carbon fiberensuring good electrical conductivity, having an outer diameter of lessthan 400 nm, preferably from 2 to 300 nm, more preferably from 1 to 80nm, and as a filler material, exhibiting good adhesive property to resinor the like, a fine carbon fiber having a form not previously known isobtained by the graphitization of a fine VGCF by a high-temperature heattreatment in the presence of a boron compound. The obtained fine carbonfiber is found to have high electrical conductivity and excellentadhesive property to resin or the like and moreover, to provide achemically and thermally stable field emission material having anexcellent electron emission property and a long life. Furthermore, thisfine carbon fiber having a novel form can be obtained by a heattreatment irrespective of the presence of a boron compound. It isconsidered that the fine carbon fiber of the present invention isfundamentally one form of carbon fibers obtained in the process ofproducing a carbon fiber having a smaller diameter and a highergraphitization degree.

[0046] The fine carbon fiber of the present invention is describedbelow.

[0047] The characteristic features of the fine carbon fiber of thepresent invention is described by referring to the attached drawings(FIGS. 1 to 4). In the Figures, the carbon sheet (a layer of graphite orcrystal close to graphite) is schematically shown by a solid line.

[0048]FIG. 1 is a cross-sectional view schematically showing aconventionally known fine carbon fiber having a diameter of less than100 nm and an aspect ratio of 10 to 15,000, where cylindrical carbonsheets are stacked one on another to form a multilayer structure(annular ring structure) and the center axis has a hollow structure. Insuch a known fine carbon fiber, the cylindrical carbon sheetsconstituting the multilayer structure all are closed with a certaincurvature at respective end parts. On the other hand, the fine carbonfiber of the present invention has the following structure.

[0049] 1) FIG. 2 and FIG. 4 each shows a view of a fine carbon fibercharacterized in that the fine carbon fiber 10 comprises cylindricalcarbon sheets stacked one on another to form a multilayer structure withthe center axis being in a hollow structure and has an outer diameter of2 to 300 nm and an aspect ratio of 10 to 15,000. At least onecylindrical carbon sheet 14, (in FIG. 2, two cylindrical carbon sheetsare shown as 14 a and 14 b and in FIG. 4, three cylindrical carbonsheets are shown, each designated as 14) among the multiple layers isfolded at the end part of the carbon fiber and continued to anothercylindrical carbon sheet 15 (in FIG. 2, two cylindrical sheets are shownas 15 a and 15 b as continued from sheets 14 a and 14 b, respectively,and in FIG. 4, three cylindrical sheets are shown, each designated as15, as continued from the three sheets 14). The cylinder formed by thefolded and continued cylindrical carbon sheets 14 and 15 (in FIG. 2,cylindrical carbon sheets 14 a and 15 a are folded and continued witheach other, and cylindrical carbon sheets 14 b and 15 b are folded andcontinued with each other) is opened at the end part of the carbonfiber. When a conventional fine carbon fiber is oxidized, the tip of thefiber is sometimes forcedly broken (see, U.S. Pat. No. 5,641,466) andsince this is not the graphite-forming condition, the carbon fiber isnot folded and continued.

[0050] 2) Referring to FIG. 2, in the fine carbon fiber of 1) above, thefolded and continued cylindrical carbon sheets 14 a and 15 a and 14 band 15 b are present at the peripheral part of the multilayer structure.

[0051] 3) Referring to FIG. 2 and the fine carbon fiber of 2) above, atleast one cylindrical carbon sheet 13 (in FIG. 2, two cylindrical carbonsheets are shown as 13 a and 13 b) closed at the end part 12A is presentinside the cylinders formed by the folded and continued cylindricalcarbon sheets (in FIG. 2, cylindrical carbon sheets 14 a and 15 a arefolded and continued with each other, and cylindrical carbon sheets 14 band 15 b are folded and continued with each other). In general, thecylinder constituted by folded and continued carbon sheets is liable tobe present in the peripheral part of the multilayer structure but in theinside thereof, a cylindrical carbon sheet is further present and theend part 12A thereof is closed in many cases.

[0052] 4) Referring to FIG. 3, in the fine carbon fiber of 3) above,cylindrical carbon sheet 11 folded at the end part and continued tocylindrical carbon sheet 12 to form a cylinder opened at the end part ofthe carbon fiber are present further inside the cylindrical carbonsheets 13 closed at the end part.

[0053] 5) Referring to FIG. 4, the fine carbon fiber comprises acylinder constructed by folded and continued carbon sheets 14 and 15.The fine carbon fiber can be comprised of only a cylinder constructed byfolded and continued carbon sheets, or as shown in FIG. 4, an unfoldedcarbon sheet 16 may be present somewhere in the inside of the cylinderconstructed by the folded and continued carbon sheets 14 and 15. Thepresence of an unfolded carbon sheet 16 somewhere in the inside of acylinder constructed by the folded and continued carbon sheets 14 and 15is not limited to the embodiment shown in FIG. 4, but can be present inother embodiments. A carbon fiber having a form opened at the end partis obtained.

[0054] 6) In a fine carbon fiber having an outer diameter of 2 to 300 nmand an aspect ratio of 10 to 15,000, the fine carbon fiber described inany one of 1) to 5) above occupies about 5% by mass or more of the finecarbon fibers.

[0055] In the foregoing, typical forms of the fine carbon fiber of thepresent invention are described, however, the fine carbon fiber of thepresent invention has at least one cylindrical carbon sheet among themultiple layers folded at the end part of the carbon fiber and continuedto another cylindrical carbon sheet and the cylinder formed by thefolded and continued cylindrical carbon sheet is opened at the end partof the carbon fiber. Other features may be freely changed. For example,the number of the cylindrical carbon sheet layer folded at the end partof the carbon fiber and continued to another cylindrical carbon sheetmay be one, but two or more adjacent cylindrical carbon sheets each maybe folded and continued to another cylindrical carbon sheet. Also, thecylindrical carbon sheets each folded and continued may or may not beadjacent to one another. For example, in FIG. 4, the cylindrical carbonsheet 14 and the cylindrical carbon sheet 15 are folded and continuedbut these are not adjacent to each other with an intervention of acylindrical carbon sheet 16 between the cylindrical carbon sheet 14 andthe cylindrical carbon sheet 15.

[0056] Furthermore, even when an amorphous carbon is present at the endpart or in the periphery of the carbon fiber constructed by the carbonsheets, the fine carbon fiber of the present invention is not affected.

[0057] With respect to the structure of the fine carbon fiber moiety,the fine carbon fiber of the present invention has a multilayerstructure where cylindrical carbon sheets comprising carbon atoms arestacked and in the center axis, a hollow cavity is present. The carbonsheet is formed of regularly oriented and continued carbon atoms andwhen observed from the longitudinal and right angled direction of thefiber, the carbon sheets are linearly multiplied almost in the fiberdirection but in some parts, the cylindrical sheet is broken anddisrupted in the longitudinal direction. Also, the center axis hollowpart may not have a constant inner diameter.

[0058] These forms of the fine carbon fiber of the present invention arenovel and have been not reported for conventional carbon fibers producedby various vapor-phase processes.

[0059] In the fine carbon fiber of the present invention, the end parthas a characteristic feature not seen in conventional carbon fibers anda portion further thinned in the end part as compared with conventionalcarbon fibers is present. As the end part is thinner, for example, inFIG. 3 the folded and continued cylindrical carbon sheets 14 and 15 hasan outer diameter of about 3 nm, the electrically conducting substancecan have clearer directivity in the electron emission and the appliedfield can be more concentrated. As a result, the substance can beimproved in field emission properties and suited as a field emissionelement. Furthermore, the fine carbon fibers are different in the shapeof the end part and therefore, when used as an electrically conductingfiller or the like, the adhesive property to resin or the like iseffectively improved.

[0060] When the fine carbon fiber of the present invention is containedin an amount of about 5% by mass or more, suitably from about 5 to about90% by mass, preferably from about 10 to about 70% by mass, morepreferably from about 10 to about 50% by mass based on the amount of thefine carbon fibers, the field emission properties are improved by virtueof the structure of the fine carbon fiber and when used as anelectrically conducting filler or the like, the adhesive property toresin or the like is effectively improved. By observation through atransmission electron microscope, the structure of the fine carbon fibercan be confirmed and when the fine carbon fiber of the present inventionis contained in an amount of about 3 to about 80% by volume, suitablyfrom about 5 to about 70% by volume, preferably from about 10 to about50% by volume based on the amount of the fine carbon fibers, the fieldemission properties are improved and when used as an electricallyconducting filler or the like, the adhesive property to resin or thelike is effectively improved.

[0061] The fine carbon fiber of the present invention has an outerdiameter of from 2 to 300 nm and an aspect ratio of from 10 to 15,000and since a long fiber can be thus obtained, the fine carbon fiber canbe added as a filler material in a large amount and an excellentreinforcement effect can be obtained.

[0062] Furthermore, the carbon fiber having the above-describedstructure allows the end parts of the peripheral carbon sheets toprotrude above the carbon sheets in the intermediate part, andtherefore, when used as an additive material for batteries, ions can beefficiently captured. In addition, the electrical conductivity is equalto that of conventional vapor grown carbon fibers and moreover, sincethe surface is not smooth, good wettability to an electrolytic solutionof a battery can be exhibited. Accordingly, the fine carbon fiber of thepresent invention is suitable as an additive material for batteries.

[0063] The fine carbon fiber having a specific form of the presentinvention may be produced by any method insofar as it is a method forproducing a fine carbon fiber having a high graphitization degree.However, a suitable method for producing the fine carbon fiber of thepresent invention is described below.

[0064] The fine carbon fiber of the present invention is generallyproduced by thermally decomposing an organic compound, particularly ahydrocarbon, using a transition metal catalyst to obtain a crude finecarbon fiber and further heat-treating the crude fine carbon fiber atabout 2,000 to about 3,500° C., preferably from about 2,500 to about3,500° C. The reason why the fine carbon fiber has the above-describedfolded structure is considered to be because the distance between carbonsheets is reduced. For example, the lattice distance (C_(o)) of thecarbon structure in which the hexagon network layers are laminated isreduced from about 0.69 nm to about 0.672-0.678 nm. Therefore, byemploying conditions to reduce the distance between carbon sheets, thefine carbon fiber of the present invention can more easily be obtained.In this regard, it is advantageous to allow a boron compound to bepresent at the time of heat-treating the crude fine carbon fiber. When aboron compound is present together with the crude fine carbon fiber, theheat-treatment temperature can be lowered by hundreds of ° C. ascompared with the case of not adding the boron compound. When the sameheat-treatment temperature is employed, the ratio of the peripheralportion to the fiber size can be increased as compared with the case ofnot adding the boron compound. The boron compound may be any substanceinsofar as it produces boron upon heating and examples thereof includeboron carbide, boron oxide and organic boron oxide. The substance may bea solid, a liquid or a gas.

[0065] At first, a crude fine carbon fiber is obtained by thermallydecomposing an organic compound, particularly a hydrocarbon, using atransition metal catalyst.

[0066] An organic transition metal compound comprises a transition metalwhich becomes a catalyst. Examples of the organic transition metalcompound include organic compounds comprising a metal belonging toGroups IVa, Va, VIa, VIIa and VIII of the periodic table. Of these,compounds such as ferrocene and nickelocene are preferred. The amount ofthe organic transition metal compound contained as the catalyst is fromabout 0.01 to about 15% by mass, preferably from about 0.03 to about 10%by mass, more preferably from about 0.1 to about 5% by mass, based onthe amount of carbon in the organic compound.

[0067] In addition, a sulfur compound is used as a co-catalyst but theform thereof is not particularly limited and any sulfur compound may beused insofar as it dissolves in an organic compound as a carbon source.Examples of the sulfur compound which can be used include thiophene,various thiols and inorganic sulfur. The amount of the sulfur compoundused is from about 0.01 to about 10% by mass, preferably from about 0.03to about 5% by mass, more preferably from about 0.1 to about 4% by mass,based on the organic compound.

[0068] Examples of the organic compound as a starting material of thecarbon fiber, which can be used, include organic compounds such asbenzene, toluene, xylene, methanol, ethanol, naphthalene, phenanthrene,cyclopropane, cyclopentane and cyclohexane, oils such as volatile oiland kerosene, gases such as CO, natural gas, methane, ethane, ethyleneand acetylene, and a mixture thereof. Of these, aromatic compounds suchas benzene, toluene and xylene are particularly preferred.

[0069] A carrier gas is employed which is usually a reducing gasincluding hydrogen gas. The carrier gas is preferably heated in advanceto about 500 to about 1,300° C. This heating is performed to produce acatalyst metal at the reaction coincidentally with the supply of carbonsource by the thermal decomposition of a carbon compound, therebycausing the reaction instantaneously, and to obtain a finer carbonfiber. When mixing the carrier gas with the starting material, if thetemperature of the heated carrier gas is less than about 500° C., thethermal decomposition of the starting material carbon compound isdifficult to occur, whereas if the temperature exceeds about 1,300° C.,the carbon fiber grows in the diameter direction and is liable to have alarge diameter.

[0070] The amount of the carrier gas used is suitably from 1 to 70 partsby mol per 1.0 part by mol of the organic compound as the carbon source.The diameter of the carbon fiber can be controlled by changing the ratiobetween the carbon source and the carrier gas.

[0071] The starting material is prepared by dissolving an organictransition metal compound and a co-catalyst sulfur compound in theorganic compound as the carbon source. The starting material in theliquid form may be sprayed as it is with the carrier gas and fed to thereaction furnace, but may also be vaporized using a part of the carriergas as a purge gas and then fed to the reaction furnace to cause areaction. In the case of obtaining a carbon fiber having a small fiberdiameter, the starting material is preferably vaporized and then fed tothe reaction furnace.

[0072] The reaction furnace used is usually a vertical electric furnace.The reaction furnace temperature is from about 800 to about 1,300° C.,preferably from about 1,000 to about 1,300° C. The starting materialsolution and the carrier gas, or the starting material gas obtained byvaporizing the starting material and the carrier gas are fed to thereaction furnace, which is elevated to a predetermined temperature, andreacted to obtain a carbon fiber.

[0073] The gas blown into the reaction furnace is thermally decomposed,whereupon the organic compound serves as a carbon source and the organictransition metal compound is formed into a particulate transition metalas a catalyst. Using the thus-formed transition metal particles asnuclei, fine carbon fibers are produced.

[0074] The obtained fine carbon fiber is further heat-treated at about900 to about 1,500° C. in an atmosphere of inert gas such as helium orargon and then heat-treated at about 2,000 to about 3,500° C., or thefine carbon fiber in the state immediately after the reaction can beheat-treated directly at about 2,000 to about 3,500° C., whereby thespecific fine carbon fiber of the present invention can be obtained.

[0075] However, the specific fine carbon fiber of the present inventioncan be more easily obtained by mixing a boron compound such as boroncarbide (B₄C), boron oxide (B₂O₃), elementary boron, boric acid (H₃BO₃)or borate, with the fine carbon fiber in the state immediately after thereaction or with the fine carbon fiber heat-treated at about 900 toabout 1,500° C. in an inert gas atmosphere, and then furtherheat-treating the mixture at about 2,000 to about 3,500° C. in an inertgas atmosphere. The amount of the boron compound added depends on thechemical or physical properties of the boron compound used and is notlimited, however, for example, in the case of using boron carbide (B₄C),the amount added is from about 0.05 to about 10% by mass, preferablyfrom about 0.1 to about 5% by mass, based on the mass of the fine carbonfiber.

[0076] The term “boron is contained in the fine carbon fiber” as usedherein means a state where boron is partially solid-dissolved andpresent on the surface of carbon fiber, between the stacked carbonhexagonal network layers or in the hollow part or where the carbon atomsare partially displaced by a boron atom.

EXAMPLE

[0077] The present invention is described in greater detail below byreferring to the Example, which is not intended to limit the scope ofthe present invention and should not be construed as doing so. Unlessindicated otherwise herein, all parts, percents, ratios and the like areby mass.

[0078] As shown in the schematic view of FIG. 5, a starting materialfeed pipe 4 for feeding a starting material vaporized in a startingmaterial vaporizer 5, and a carrier gas feed pipe 6 were connected tothe top of a vertical heating furnace 1 (inner diameter: 170 mm, length:1,500 mm). The vertical heating furnace 1 is provided with one or moreheaters 2 for heating the furnace and with a starting material recoverysystem 3.

[0079] A toluene solution having dissolved therein 3% by mass offerrocene and 1% by mass of thiophene was vaporized and fed through thestarting material feed pipe 4 at a rate of 20 g/min and hydrogen as acarrier gas was fed at a rate of 75 l/min, thereby performing thereaction. FIG. 6 shows a transmission electron microscope photograph ofthe fine carbon fiber obtained by this reaction.

[0080] The fine carbon fiber obtained by the reaction was heat-treatedat 1,300° C. in an Ar (argon) atmosphere and the fiber treated at 1,300°C. was further heat-treated at 2,800° C. in an Ar atmosphere to obtain afine carbon fiber in a recovery by mass of 96%. The lattice distance(C_(o)) is 0.68-0.675 nm.

[0081] Separately, the fine carbon fiber heat-treated above at 1,300° C.in an Ar atmosphere was mixed with 4% by mass as B₄C and thenheat-treated at 2,800° C. in an Ar atmosphere to obtain a fine carbonfiber in a recovery by mass of 94%. The lattice distance (C_(o)) is0.672 nm. FIG. 7 shows a transmission electron microscope photograph ofthe obtained carbon fiber.

[0082] In both FIG. 6 and FIG. 7, a multilayer structure is shown wherecylindrical carbon sheets comprising carbon atoms are stacked one onanother, and the center axis thereof has a hollow structure. However, inFIG. 6, the end part is closed as in the schematic view of FIG. 1,whereas in FIG. 7, a multilayer structure having a form corresponding tothe schematic view of FIG. 3 is shown.

[0083] More specifically, in FIG. 7, the fine carbon fiber has aperipheral part (corresponding to 14, 15 of FIG. 3) forming a cylinderhaving an opened end, an intermediate part (corresponding to 13 of FIG.13) having a closed end, and inside the intermediate part, a cylinder(corresponding to 11, 12 of FIG. 3) having an opened end. The carbonsheets (corresponding to 14, 15 and 11, 12 of FIG. 3) constituting theouter side and the inner side of the multilayer structure, respectively,are each folded at the terminal end and connected to each other. Thecarbon sheets (corresponding to 13 of FIG. 3) lying in the intermediatepart between the folded carbon sheets in the outer side and the innerside of the multilayer structure have a closed end (corresponding to 12Aof FIG. 3). In FIG. 7, a carbon sheet is observed in the cross-sectionaldirection of the fiber at the end part of the carbon fiber. However,this is not the carbon sheet and the folded part of the carbon sheet inthe peripheral part is viewed like a carbon sheet and the carbon sheetis not present in the axial center part of the fiber. Similarly, in FIG.7, in the end part of the carbon fiber, it appears as if the upper partof the carbon sheet at the closed end part of the intermediate carbonsheet is not a cavity but some substance is present, but this is anamorphous carbon attached to that portion and has no relationship withthe structure of the carbon fiber. It is seen that amorphous carbon ispresent also on the surface in the circumferential part of the carbonfiber.

[0084] At this time, fibers having an outer diameter of about 10 toabout 100 nm and an aspect ratio of tens or more were produced.Furthermore, when observed through a transmission electron microscope,fibers having the above-described characteristic features were found tooccupy half the number or more (hereinafter “half the number or moremeans “60% or more by number”).

[0085] According to the present invention, unlike conventional carbonfibers or vapor grown carbon fibers, the fine carbon fiber obtained ischaracterized in that the outer diameter is from 2 to 300 nm, suitablyfrom 5 to 200 nm, and preferably from 10 to 100 nm, the aspect ratio isfrom 10 to 15,000, suitably from 10 to 5000, and preferably from 20 to1,000, at least one cylindrical carbon sheet among the multiple layersis folded at the end part of the carbon fiber and continued to anothercylindrical carbon sheet, and the folded and continued cylindricalcarbon sheets form a cylinder opened at the end part. This fine carbonfiber is useful as an electrically conducting filler for the fieldemission, gas occlusion using, for example, H₂, CH₄, C₂H₄, etc, orresin.

[0086] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A fine carbon fiber comprising a multilayerstructure comprising stacked cylindrical carbon sheets and a center axishaving a hollow structure, wherein the fine carbon fiber has an outerdiameter of 2 to 300 nm and an aspect ratio of 10 to 15,000, and whereinat least one cylindrical carbon sheet layer among the multiple layers isfolded at an end part of the carbon fiber and continued to anothercylindrical carbon sheet and the folded and continued cylindrical carbonsheets form a cylindrical structure opened at the end part.
 2. The finecarbon fiber as claimed in claim 1, wherein the folded and continuedcylindrical carbon sheets are present in a peripheral part of themultilayer structure.
 3. The fine carbon fiber as claimed in claim 1 or2, wherein a cylindrical carbon sheet closed at the end part is presentinside the cylindrical structure formed by the folded and continuedcylindrical carbon sheets.
 4. The fine carbon fiber as claimed in claim3, wherein cylindrical carbon sheets folded and continued at the endpart to form a cylinder opened at the end part of the carbon fiber arepresent further inside the cylindrical carbon sheet closed at the endpart.
 5. A fine carbon fiber having an outer diameter of 2 to 300 nm andan aspect ratio of 10 to 15,000, wherein the fine carbon fiber accordingto claim 1 comprises about 5% by mass or more of the fine carbon fibers.6. A fine carbon fiber having an outer diameter of 2 to 300 nm and anaspect ratio of 10 to 15,000, wherein the fine carbon fiber according toclaim 1 comprises about 5 to about 90% by mass of the fine carbon fiberin the fine carbon fibers.
 7. A fine carbon fiber having an outerdiameter of from 2 to 300 nm and an aspect ratio of from 10 to 15,000,wherein the fine carbon fiber according to claim 1 occupies from about 3to 80% by volume in the fine carbon fibers when observed through atransmission electron microscope.
 8. The fine carbon fiber as claimed inclaim 1, wherein the fine carbon fiber is vapor grown carbon fiber. 9.The fine carbon fiber as claimed in claim 1, wherein the carbon fibercomprises a boron atom.
 10. The fine carbon fiber as claimed in claim 1,wherein carbon atoms of the carbon fiber are partially displaced byboron atoms.
 11. A process for producing a fine carbon fiber comprisinga multilayer structure comprising stacked cylindrical carbon sheets anda center axis having a hollow structure, wherein the fine carbon fiberhas an outer diameter of 2 to 300 nm and an aspect ratio of 10 to15,000, and wherein at least one cylindrical carbon sheet layer amongthe multiple layers is folded at the end part of the carbon fiber andcontinued to another cylindrical carbon sheet and the folded andcontinued cylindrical carbon sheets form a cylindrical structure openedat the end part, said process comprising: heat-treating a fine carbonfiber having an outer diameter of 2 to 300 nm and an aspect ratio of 10to 15,000, and having a multilayer structure formed by cylindricalcarbon sheets stacked one on another, with the center axis having ahollow structure.
 12. The process for producing the fine carbon fiber asclaimed in claim 11, wherein the heat-treatment temperature is fromabout 2,000 to about 3,500° C.
 13. The process for producing the finecarbon fiber as claimed in claim 11 or 12, further comprising mixing aboron compound with the carbon fiber that is subjected to the heattreatment.
 14. The process according to claims 11, 12 or 13, wherein thefine carbon fiber that is subjected to the heat treatment is formed byvapor phase thermal decomposition of an organic compound, using atransition metal catalyst.